In recent years, by taking advantage of such features as light weight, small thickness, and low power consumption, liquid crystal display devices have been widely used for display devices in information devices such as notebook-type personal computers and word processors, or for display devices in video devices such as portable televisions, camcorders, and vehicle navigation systems. In order to realize a bright display screen, many of these liquid crystal display devices have such a configuration that illuminating light is applied from the rear of a display element by a built-in lighting unit.
For this lighting unit, an edge light system, in which a light guiding plate is placed on the rear of a display element (liquid crystal display panel) and a line light source of a fluorescent discharge tube or the like is disposed on an end face of the light guiding plate, is advantageous in thickness reduction and brightness uniformity across the light-emitting surface. Accordingly, in many cases the edge light system is adopted for the backlight system of a liquid crystal display device used for notebook-type personal computers or the like since a smaller thickness is given a priority. In addition, a liquid crystal display device used for portable televisions, vehicle navigation systems, or the like, adopts in many cases an edge light system in which light sources are arranged on four end faces or two end faces of a rectangular-shaped light guiding plate in order to obtain both brightness and small thickness.
FIG. 5 is a cross-sectional view showing the configuration of a conventional edge-light type lighting unit and a liquid crystal display device using the same, and FIG. 6 is a cross-sectional view taken along line VI—VI of FIG. 5. In FIG. 5, a liquid crystal display device 301 is oriented as shown in the figure according to the conditions of use.
Referring to FIG. 5, this liquid crystal display device 301 has a transparent light guiding plate 3, shaped like a rectangular flat panel, that transmits light in a direction parallel to a principal surface. On the rear face of the light guiding plate 3, a reflecting sheet 5 is disposed. Further, respective light sources 2 are arranged along the upper end face and the lower end face of the light guiding plate 3. That is, this lighting unit adopts an edge light system in which light sources are arranged on two opposing end faces. In addition, respective reflectors 4 having a substantially U-shaped cross section are arranged so as to surround this pair of light sources 2. The light sources 2, the light guiding plate 3, the reflectors 4, and the reflecting sheet 5 constitute a lighting unit.
Additionally, a rear cover RC is disposed so as to be in contact with the rear faces of the reflecting sheet 5 and the reflectors 4, and respective housings 6 are disposed on the upper and lower ends of this rear cover RC. The housing 6 is disposed so as to surround the end face and the front face of the reflector 4 and the edge portion of the front face of the light guiding plate 3 and to retain the light guiding plate 3 by being partially interposed between the reflector 4 and the light guiding plate 3. These portions 6a that retain the light guiding plate 3 (hereafter referred to as “light guiding plate-retaining portions”) each have a rectangular window portion 101 in the middle, as shown in FIG. 6. The rear cover RC and the pair of housings 6, 6, arranged on upper and lower ends of the rear cover RC, constitute a case of the liquid crystal display device 301.
A protruding portion 6b is formed on an edge of each front face of the pair of housings 6, 6, and a liquid crystal display panel (liquid crystal display element) 1 is disposed so as to span between the front faces of the two housings 6, 6. In addition, a pair of front frames FC are disposed so as to be in contact with the protruding portions 6b of the respective housings 6 and to cover end faces of the housings 6 and edge portions of the liquid crystal display panel 1. This pair of front frames FC constitute a cover portion of the case of the liquid crystal display device 301. The liquid crystal display device 301 also incorporates a driver circuit, which is not shown in the figure, for the liquid crystal display panel 1.
The light guiding plate 3 is formed of a material such as an acrylic resin. The reflector sheet 5 is formed by, for example, a film made of a light-reflective white resin. The housing 6 is composed of, for example, a polycarbonate resin.
In the liquid crystal display device 301 thus configured, although thin, the light source 2 and the reflector 4 are retained by the housings 6 and the light guiding plate 3 is suitably retained by the light guiding plate-retaining portions 6a. 
The light emanating from the light sources 2 passes through the window portions 101 of the light guiding plate-retaining portions 6a of the housings 6 and enters the end faces of the light guiding plate 3 either directly or with it being reflected by the reflectors 4; then it propagates in the light guiding plate 3 and emanates from its front face. During this process, the light that leaks from the rear face of the light guiding plate 3 is returned into the light guiding plate 3 by the reflecting sheet 5. In addition, scattering dots are provided on the rear face of the light guiding plate 3 at densities corresponding to the distances from the light source 2, whereby the distribution of intensity of the light emanating from the light guiding plate 3 becomes uniform. The light emanating from the light guiding plate 3 passes through the liquid crystal display panel 1, where the transmittance thereof is controlled by the driver circuit according to an image signal, and thereby an image is displayed on a display screen 1a of the liquid crystal display panel 1.
This thin-type liquid crystal display device 301, however, suffers from brightness unevenness such that the light guiding plate-retaining portions 6a block the light that is to enter the end faces of the light guiding plate 3, causing the portions where the light is blocked to appear as dark shades on the display screen 1a. This is particularly conspicuous in an edge light system in which the light sources 2 are arranged in a substantially L-shape along two adjoining end faces of the light guiding plate 3. The reason is that, due to the shape of the light guiding plate-retaining portion 6a of the housing 6, a large proportion of the light that is to enter the light guiding plate 3 is blocked by the light guiding plate-retaining portion 6a. 